This invention relates to microfilm readers and more particularly to microfilm readers having a carriage assembly which can be precisely indexed and held in predetermined positions.
A microfiche is usually a small piece of film, approximately 4 inches by 6 inches, having a plurality of images photographed thereon in an orthogonal array. To view the images, it is necessary for the fiche to be placed within a fiche carriage and moved about orthogonally relative to an optical system to project the desired image on the film to a screen.
Microfilm readers normally project an image to a viewing screen in one of two manners. The first is to project an image on to an opaque viewing screen and view the image from the front of the screen. This is called a front projection screen and one example is illustrated in U.S. Pat. No. 4,130,352. The second method is through the use of a rear projection screen which is a translucent piece of plastic having optical characteristics which intercept the projected image and display it on the opposite side of the screen. Thus, the image is not viewed from the side of the screen from which it is projected, but rather from the opposite side of the projection system. An example of this is illustrated in U.S. Pat. No. 4,050,799.
The fiche carriage generally moves about freely so that individual images can be placed in the projection system. With microfilm gaining greater acceptability and wider usage it has become a method of distributing varied types of information. For example, sales people can carry a portable microfilm reader into a customer's location and give a complete presentation utilizing relatively few microfiche. Some microfilm viewers are designed as rear projection units with the person giving the presentation and controlling the fiche carriage on one side of the viewing screen (projection side) while the customer or viewer is on the opposite side (viewing side) of the screen. The operator of the fiche carriage might not be able to see the images which are being projected on to the screen without peering around the corner of the screen to see the images. This becomes a nuisance when trying to give a sales talk coordinated with microfilm. The person giving the presentation knows the location of the desired specific images on the microfiche. If the microfiche can be accurately positioned on the fiche carriage and the fiche carriage has the capability to be precisely incrementally moved, then there would be no need for the operator to look around at the screen and see the image that is being projected and its position. The operator would already know that the image is accurately positioned in the projection path and on the screen. Different images could easily be presented by incrementally moving the carriage to a different held position.
Therefore, it is an object of the present invention to provide a microfilm reader having a carriage assembly which can accurately and precisely be held in predetermined positions as it moves orthogonally on the microfilm viewer.
Another object of the invention is to provide a carriage assembly having magnetic means to magnetically hold the carriage assembly in precise and accurate positions such that a desired image on the microfilm is displayed on the viewing screen. A further object is to provide a microfiche carriage indexing system which is subject to a minimum amount of wear and is also economical to manufacture.